From: Bharata B Rao <[email protected]>
namespace_sem is a rwsem. It is acquired as read sem at only one place(used
by /proc/mounts, /proc/<pid>/mounts and /proc/<pid>/mountstats). In all other
cases it is acquired as a write sem. So, as there is not more than one reader
for this sem, this can be a generic sem (and not rwsem) and if so it can be
a mutex.
Patch is for 2.6.22-rc1-mm1.
Signed-off-by: Bharata B Rao <[email protected]>
---
fs/namespace.c | 48 ++++++++++++++++++++++++------------------------
1 files changed, 24 insertions(+), 24 deletions(-)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -37,7 +37,7 @@ static int event;
static struct list_head *mount_hashtable __read_mostly;
static int hash_mask __read_mostly, hash_bits __read_mostly;
static struct kmem_cache *mnt_cache __read_mostly;
-static struct rw_semaphore namespace_sem;
+static struct mutex namespace_mutex;
int nr_user_mounts;
int max_user_mounts = 1024;
@@ -396,7 +396,7 @@ static void *m_start(struct seq_file *m,
struct list_head *p;
loff_t l = *pos;
- down_read(&namespace_sem);
+ mutex_lock(&namespace_mutex);
list_for_each(p, &n->list)
if (!l--)
return list_entry(p, struct vfsmount, mnt_list);
@@ -413,7 +413,7 @@ static void *m_next(struct seq_file *m,
static void m_stop(struct seq_file *m, void *v)
{
- up_read(&namespace_sem);
+ mutex_unlock(&namespace_mutex);
}
static inline void mangle(struct seq_file *m, const char *s)
@@ -683,7 +683,7 @@ static int do_umount(struct vfsmount *mn
return retval;
}
- down_write(&namespace_sem);
+ mutex_lock(&namespace_mutex);
spin_lock(&vfsmount_lock);
event++;
@@ -696,7 +696,7 @@ static int do_umount(struct vfsmount *mn
spin_unlock(&vfsmount_lock);
if (retval)
security_sb_umount_busy(mnt);
- up_write(&namespace_sem);
+ mutex_unlock(&namespace_mutex);
release_mounts(&umount_list);
return retval;
}
@@ -1002,12 +1002,12 @@ static int do_change_type(struct nameida
if (nd->dentry != nd->mnt->mnt_root)
return -EINVAL;
- down_write(&namespace_sem);
+ mutex_lock(&namespace_mutex);
spin_lock(&vfsmount_lock);
for (m = mnt; m; m = (recurse ? next_mnt(m, mnt) : NULL))
change_mnt_propagation(m, type);
spin_unlock(&vfsmount_lock);
- up_write(&namespace_sem);
+ mutex_unlock(&namespace_mutex);
return 0;
}
@@ -1030,7 +1030,7 @@ static int do_loopback(struct nameidata
if (err)
return err;
- down_write(&namespace_sem);
+ mutex_lock(&namespace_mutex);
err = -EINVAL;
if (IS_MNT_UNBINDABLE(old_nd.mnt))
goto out;
@@ -1062,7 +1062,7 @@ static int do_loopback(struct nameidata
}
out:
- up_write(&namespace_sem);
+ mutex_unlock(&namespace_mutex);
path_release(&old_nd);
return err;
}
@@ -1124,7 +1124,7 @@ static int do_move_mount(struct nameidat
if (err)
return err;
- down_write(&namespace_sem);
+ mutex_lock(&namespace_mutex);
while (d_mountpoint(nd->dentry) && follow_down(&nd->mnt, &nd->dentry))
;
err = -EINVAL;
@@ -1176,7 +1176,7 @@ static int do_move_mount(struct nameidat
out1:
mutex_unlock(&nd->dentry->d_inode->i_mutex);
out:
- up_write(&namespace_sem);
+ mutex_unlock(&namespace_mutex);
if (!err)
path_release(&parent_nd);
path_release(&old_nd);
@@ -1238,7 +1238,7 @@ int do_add_mount(struct vfsmount *newmnt
{
int err;
- down_write(&namespace_sem);
+ mutex_lock(&namespace_mutex);
/* Something was mounted here while we slept */
while (d_mountpoint(nd->dentry) && follow_down(&nd->mnt, &nd->dentry))
;
@@ -1267,11 +1267,11 @@ int do_add_mount(struct vfsmount *newmnt
list_add_tail(&newmnt->mnt_expire, fslist);
spin_unlock(&vfsmount_lock);
}
- up_write(&namespace_sem);
+ mutex_unlock(&namespace_mutex);
return 0;
unlock:
- up_write(&namespace_sem);
+ mutex_unlock(&namespace_mutex);
mntput(newmnt);
return err;
}
@@ -1337,9 +1337,9 @@ static void expire_mount_list(struct lis
get_mnt_ns(ns);
spin_unlock(&vfsmount_lock);
- down_write(&namespace_sem);
+ mutex_lock(&namespace_mutex);
expire_mount(mnt, mounts, &umounts);
- up_write(&namespace_sem);
+ mutex_unlock(&namespace_mutex);
release_mounts(&umounts);
mntput(mnt);
put_mnt_ns(ns);
@@ -1612,12 +1612,12 @@ static struct mnt_namespace *dup_mnt_ns(
init_waitqueue_head(&new_ns->poll);
new_ns->event = 0;
- down_write(&namespace_sem);
+ mutex_lock(&namespace_mutex);
/* First pass: copy the tree topology */
new_ns->root = copy_tree(mnt_ns->root, mnt_ns->root->mnt_root,
CL_COPY_ALL | CL_EXPIRE, 0);
if (IS_ERR(new_ns->root)) {
- up_write(&namespace_sem);
+ mutex_unlock(&namespace_mutex);
kfree(new_ns);
return NULL;
}
@@ -1651,7 +1651,7 @@ static struct mnt_namespace *dup_mnt_ns(
p = next_mnt(p, mnt_ns->root);
q = next_mnt(q, new_ns->root);
}
- up_write(&namespace_sem);
+ mutex_unlock(&namespace_mutex);
if (rootmnt)
mntput(rootmnt);
@@ -1850,7 +1850,7 @@ asmlinkage long sys_pivot_root(const cha
user_nd.mnt = mntget(current->fs->rootmnt);
user_nd.dentry = dget(current->fs->root);
read_unlock(¤t->fs->lock);
- down_write(&namespace_sem);
+ mutex_lock(&namespace_mutex);
mutex_lock(&old_nd.dentry->d_inode->i_mutex);
error = -EINVAL;
if (IS_MNT_SHARED(old_nd.mnt) ||
@@ -1905,7 +1905,7 @@ asmlinkage long sys_pivot_root(const cha
path_release(&parent_nd);
out2:
mutex_unlock(&old_nd.dentry->d_inode->i_mutex);
- up_write(&namespace_sem);
+ mutex_unlock(&namespace_mutex);
path_release(&user_nd);
path_release(&old_nd);
out1:
@@ -1951,7 +1951,7 @@ void __init mnt_init(unsigned long mempa
int i;
int err;
- init_rwsem(&namespace_sem);
+ mutex_init(&namespace_mutex);
mnt_cache = kmem_cache_create("mnt_cache", sizeof(struct vfsmount),
0, SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL, NULL);
@@ -2008,11 +2008,11 @@ void __put_mnt_ns(struct mnt_namespace *
LIST_HEAD(umount_list);
ns->root = NULL;
spin_unlock(&vfsmount_lock);
- down_write(&namespace_sem);
+ mutex_lock(&namespace_mutex);
spin_lock(&vfsmount_lock);
umount_tree(root, 0, &umount_list);
spin_unlock(&vfsmount_lock);
- up_write(&namespace_sem);
+ mutex_unlock(&namespace_mutex);
release_mounts(&umount_list);
kfree(ns);
}
On 5/17/07, Bharata B Rao <[email protected]> wrote:
> From: Bharata B Rao <[email protected]>
>
> namespace_sem is a rwsem. It is acquired as read sem at only one place(used
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Actually, this ...
> by /proc/mounts, /proc/<pid>/mounts and /proc/<pid>/mountstats). In all other
> cases it is acquired as a write sem. So, as there is not more than one reader
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
... does not mean this. Multiple threads could be reading mounts or
mountstats, and otoh mount(2) and umount(2) (acquire it as write sem)
could be less frequent?
> for this sem, this can be a generic sem (and not rwsem) and if so it can be
> a mutex.
On Thu, May 17, 2007 at 10:20:41AM +0530, Bharata B Rao wrote:
> From: Bharata B Rao <[email protected]>
>
> namespace_sem is a rwsem. It is acquired as read sem at only one place(used
> by /proc/mounts, /proc/<pid>/mounts and /proc/<pid>/mountstats). In all other
> cases it is acquired as a write sem. So, as there is not more than one reader
> for this sem, this can be a generic sem (and not rwsem) and if so it can be
> a mutex.
Except that read accesses outnumber write ones by far and we have no reason
for serializing them against each other. NAK.
On Thu, May 17, 2007 at 06:00:46AM +0100, Al Viro wrote:
> On Thu, May 17, 2007 at 10:20:41AM +0530, Bharata B Rao wrote:
> > From: Bharata B Rao <[email protected]>
> >
> > namespace_sem is a rwsem. It is acquired as read sem at only one place(used
> > by /proc/mounts, /proc/<pid>/mounts and /proc/<pid>/mountstats). In all other
> > cases it is acquired as a write sem. So, as there is not more than one reader
> > for this sem, this can be a generic sem (and not rwsem) and if so it can be
> > a mutex.
>
> Except that read accesses outnumber write ones by far and we have no reason
> for serializing them against each other. NAK.
Ok, I wasn't aware that this rwsem had many concurrent readers. From whatever
little I could see (during system boot and during kernel compilation with
make -j), the number of concurrent readers very rarely touched 3 and was 1
mostly. May be there are other concurrent readers of this sem. Any idea
from where they could be coming from ?
Regards,
Bharata.
On Thu, May 17, 2007 at 12:47:20PM +0530, Bharata B Rao wrote:
> Ok, I wasn't aware that this rwsem had many concurrent readers. From whatever
> little I could see (during system boot and during kernel compilation with
> make -j), the number of concurrent readers very rarely touched 3 and was 1
> mostly. May be there are other concurrent readers of this sem. Any idea
> from where they could be coming from ?
Anybody reading from /proc/mounts? You do realize that on systems where
/etc/mtab is a symlink to /proc/mounts every df(1), etc. - anything that
wants to find out all mountpoints - will read through that file?
The whole point is that we need exclusion between those who modify
mount tree *and* between them and anyone who looks at the mount tree.
We don't need any exclusion between several processes that happen to
examine the current mount tree at the same time. And since those
are far more frequent than those who modify the tree...